Capacitive sensing apparatus are widely used in electronic devices such as smart phones, tablet computers, wearable devices, and fingerprint scanners. Examples of capacitive sensing apparatus include touch panels (or touch screens) and fingerprint sensors.
Touch panels typically include a touch-sensitive surface. When a stylus or a human body part, such as a finger, touches a point on the surface, the touch position is recognized and processed. Upon this principle, a user may make a selection or a gesture.
One type of touch panels, capacitive touch panel using mutual capacitance sensing technology, has gained popularity due to its capability of sensing multiple touch points (or multi-touch) simultaneously. A mutual capacitance sensing touch panel typically includes two conductive layers separated by a dielectric layer. The two conductive layers may be made of a transparent conductive material such as indium tin oxide (ITO). The two conductive layers each include a plurality of conductors oriented in a particular direction. A mutual capacitance forms when one conductor in one layer overlays another conductor in the other layer. In one exemplary panel, one layer includes M row conductors in the horizontal direction and the other layer includes N column conductors in the vertical direction so as to form a matrix of M×N mutual capacitances in the panel, one in each intersection. When a finger touches the panel, human body capacitance to ground effectively alters the mutual capacitance at the touch point, which can be detected to indicate the touch position.
Capacitive fingerprint sensors may include a single row of sensors (e.g. sweep scanners) or a two-dimensional array of sensors (e.g. area scanners). Each sensor typically includes an active capacitive feedback circuit whose effective capacitance is decreased by the presence of a finger near the sensor. The amount of capacitance decrease is more for ridges and less for valleys, thereby allowing the user's fingerprint to be recorded or recognized.
A device using the above capacitive sensing apparatus (e.g. a touch panel or a fingerprint sensor) further includes driving and sensing circuits that drive signals to and sense outputs from the apparatus in order to detect the touch events. For example, a stimulus in the form of a square wave or a sine wave is driven onto a driving channel (e.g. a row conductor in a touch panel). This stimulus is coupled onto a sensing channel (e.g. a column conductor in a touch panel) through a capacitance between the driving and sensing channels. The outputs from the sensing channels are monitored (or sensed) for detecting the touch events. When a finger touches the apparatus, one or more of the outputs from the sensing channels will change in magnitude, indicating the touch positions in the case of touch panels, or the touch impression in the case of fingerprint sensors.
However, noises can easily interfere with the stimulus and/or the outputs, causing sensing errors. For example, noises may come from nearby environment having wireless signals such as 802.11 and Bluetooth, a switched-mode power supply, and the like. To increase sensing accuracy, common methods increase the amplitude of the stimulus or increase the time period for sensing. However, increasing the stimulus amplitude requires increased dynamic range in the circuits, and increasing the time period slows down the sensing operation. Either method also results in higher power consumption.
Accordingly, what is needed is improvement in the driving and sensing circuits associated with the capacitive sensing apparatus.